By "pigments", natural or synthetic, organic or mineral substances are generally meant, presenting themselves in the form of small particles which are practically insoluble in their application medium and which are used for their colouring, protective and/or magnetic properties. For their part, the "fillers" are generally used, in the first place, as filling materials which make it possible, at a lower cost, to preserve the essential applicable properties of their application medium, indeed to improve certain of these properties.
These notions do not in any way exclude one another since numerous substances qualify at the same time as "pigments" and "fillers", depending on the fields and conditions of application envisaged. This is the case for example with clays (kaolins, smectites and others), calcium carbonates (precipitated or natural), talc, titanium dioxide, calcium or barium sulphates or sulfoaluminates
Numerous fields of application widely use pigments and fillers, and in particular the industries manufacturing paper, paints and varnishes, inks, adhesives, detergents, textile materials and leathers, plastics materials and rubbers, films, ceramics and enamels, construction materials and cosmetic materials.
In certain industries, certain pigments or fillers are likewise called "extenders" "extender pigments", "pigment fillers" or "pigment coatings", these labels being included here in the all-embracing notion of "pigments and/or fillers".
The pigments and the fillers present very great diversity as much in terms of origin and nature as in physico-chemical and application properties as emerges, amongst other things, from the work entitled "ULLMANN'S ENCYCLOPEDIA OF INDUSTRIAL CHEMISTRY" --Fifth Edition --VCH, in particular the following passages of said work:
Volume A7 (1986): "Clays", pages 109-136, PA0 Volume A18 (1991): "Paints and Coatings", in particular pages 365, 437, 446 and 455-465, PA0 Volume A18 (1991): "Paper and Pulp", in particular pages 611-614 and 647-658, PA0 Volume A20 (1992): "Plastics, Additives", in particular pages 495-500, PA0 Volume A20 (1992): "Pigments, Inorganic", pages 245, 250-257 and 266-271, and "Pigments, Organic", pages 371-377, PA0 Volume A22 (1993): "Printing Inks", in particular pages 154-156, and PA0 Volume A23 (1993): "Rubber, 4. Chemicals and Additives", in particular pages 391-399.
As far as more specifically applications to do with paper are concerned, one can refer to the work "Pigments for Paper",1997, TAPPI PRESS, pages 1-20. For the applications in paints, reference can be made to the work "Peintures en phase aqueuse (Paints in an aqueous phase)" Chapter 9 "Pigments et matieres de charge (Pigments and Filling Materials)",1997, GALVANO ORGANO.
In a certain number of uses it is, if not obligatory, at least advantageous to be able to have available pigments and/or fillers in the form of aqueous dispersions, commonly called "slurries", presenting variable amounts of solid matter ("S.M."), generally greater than 20-25%, for example in the order of 35-70% for those intended for paper coating.
The respective advantages and drawbacks of the slurries and of the dry forms of pigments are summarised on page 72 of the above-quoted work "Pigments for paper". Compared with dry forms, the aqueous dispersions generally make it possible to improve the industrial operations of metering, conveying and mixing. They likewise permit considerable time saving to the users, especially when they come in the form of ready-to-use products.
These slurries likewise constitute one of the means making it possible to add all or part of the water which is necessary for the desired constitution, reactivity and evolution of the application medium in which pigment(s) and/or filler(s) are introduced (for example a coating mix for paper), even if later said water is eliminated more or less completely at the stage of the final product (for example dried coated paper).
The preparation, storage, handling and use of dispersions of pigments or fillers pose however technical and economic problems. First of all, obtaining and keeping homogenous and stable dispersions necessitate using mechanical means (industrial agitators) and chemical means (dispersing agents, biocides) which increase the cost price of these compositions. Now, industrially, these means are obligatory even if there is a risk of altering significantly the properties, especially optical properties, of the finished product (paper, paint, etc.).
The dispersing agents commonly used are of polyanionic nature such as the salts of polyphosphoric or polyacrylic acids. However the polyphosphates are liable to precipitate and thus to become ineffective in a few days, particularly when the dispersion is stored at a relatively high temperature or at a very alkaline pH. Moreover, the optimum rate of introduction of a dispersing agent into a slurry is difficult to determine in a rapid and reliable manner. In particular, the excessive dose of a dispersing agent can result in the undesirable increase in viscosity of a slurry. Furthermore, in certain applications, the dispersing agent contained in a slurry of acceptable viscosity can increase unfavourably the viscosity of the final application medium, for example a coating composition for paper based on anionic binding agents.
The dispersibility and the ability to be ground of a pigment such as titanium dioxide can be improved or controlled by surfacing with numerous mineral or organic, hydrophilic or hydrophobic compounds, as described in the works already quoted, "Pigments for Paper" (pages 163-165) and "ULMANN'S ENCYCLOPEDIA CHEMISTRY" (pages 279, 282 --Volume A20). Among these compounds are cited the polymers of alcohols, the amines and the organic acids which are known moreover as agents for milling cement. Patent FR 2.146.484 discloses likewise the possibility of dispersing directly filtered/dehydrated titanium dioxide with agents such as polyols (2-amino-2 ethyl 1-propanol, sorbitol, mannitol), triethanolamine or polycarboxylates, with a view to its use in paints. However, from reading this patent it emerges that, a) to be effective, these agents have to be introduced in very precise proportions, i.e. between 0.3 and 0.75% by weight of dry pigment, and b) these agents do not in any way stop the dispersions of titanium dioxide obtained from evolving unfavourably during storage, the viscosity of said dispersions increasing from 29% to 227% in a week, as emerges from the data of table I of this patent.
Another drawback of the aqueous dispersions of pigment(s) and/or filler(s) resides in the additional cost of energy linked to the transport then to the elimination of the water which they contain. It is thus generally sought to be able to increase the solid matter (SM). Furthermore, slurries with high SM are desired to increase the productivity of certain industrial installations such as paper coating machines.
However, increasing the concentration of a slurry is often limited indeed impossible because of the occurrence of induced phenomena of increasing the viscosity and/or the aptitude for dilatancy, these phenomena being able to have a very unfavourable effect on the rheological characteristics, in particular the pumpability, of the slurry.
Moreover, many pigments show a marked propensity to dry on the walls of the storage vats of the slurries and to form there undesirable deposits, notably ones favourable to the proliferation of microbes. Such a proliferation can very seriously limit the possibilities of use of the pigments but likewise of the final products containing them, as described in the article entitled "Microbiological Control of Pigments and Fillers in Paper Industry" published in "The Fundamentals of Papermaking Material", Volume 2, pp 955-993, C. F. BAKER.
It emerges from the above that there was a need to have available aqueous dispersions of pigment(s) and/or filler(s) which present simultaneously:
good rheological stability in storage, PA1 good microbiological stability in storage, PA1 low tendency to form deposits in the preparation/storage enclosures and in transport, particularly after storage, PA1 low aptitude for dilatancy and good pumpability, especially after storage, and all whilst being capable of presenting if necessary high amounts of SM, including ones greater than those used in current industrial practice. PA1 from 0.5 to 100%, preferably from 5 to 100% and more especially from 10 to 90%, by weight of maltitol, and PA1 from 0 to 99.5%, preferably from 0 to 95% and more especially from 10 to 90% by weight of sorbitol and/or mannitol.
And the applicants have found, after extensive research, that such a result could be obtained as soon as the dispersions in question confirmed a certain number of characteristics in their composition and rheology, said characteristics being connected to the use, as an additive, of particular saccharide compositions.